code: def time_series():
      n = 10**7
      signal = np.random.normal(0, 0.3, size=n).cumsum() + 50
  
      def noise(var, bias, n):
          return np.random.normal(bias, var, n)
  
      ys = signal + noise(1, 10 * (np.random.random() - 0.5), n)
  
      df = pd.DataFrame({"y": ys})
      df["x"] = df.index
      return df
  
file_path: datashader/datashader/tests/benchmarks/test_canvas.py
imports: import pytest
  import os
  import numpy
  import pandas
  import datashader
label_desc: Use image node_id 1 for calling a global function with example usage: time_series() and returns: df, np